A Voice Activity Detector (VAD) determines whether an input signal contains speech or background noise. A typical application for a VAD is in wireless communication systems, in which the voice activity detection can be used for controlling a discontinuous transmission system, where transmission is inhibited when speech is not detected. A VAD can also be used in e.g. echo cancellation and noise cancellation.
Various methods for voice activity detection are known in prior art. The main problem is to reliably detect speech from background noise in noisy environments. Patent publication U.S. Pat. No. 5,459,814 presents a method for voice activity detection in which an average signal level and zero crossings are calculated for the speech signal. The solution achieves a method which is computationally simple, but which has the drawback that the detection result is not very reliable. Patent publications WO 95/08170 and U.S. Pat. No. 5,276,765 present a voice activity detection method in which a spectral difference between the speech signal and a noise estimate is calculated using LPC (Liner Prediction Coding) parameters. These publications also present an auxiliary VAD detector which controls updating of the noise estimate. The VAD methods of all the above mentioned publications have problems to reliably detect speech when speech power is low compared to noise power.